Residual gases coming from production of nitric acid, apparatus for pickling of metals with acids, thermal decomposition of nitrates, etc. contain a high concentration of nitrogen oxides NO.sub.2, which are polluting compounds because of their corrosive action and the contamination of the air that they cause.
To combat industrial pollution it is necessary to have means for eliminating toxic wastes, including nitrogen oxides produced in combustions of any origin.
It has been known for seveal decades, especially from the teaching of German patent No. 1,088,938, that oxygen is eliminated during catalytic reduction of nitrogen oxides with fuels. With this process, fuel consumption is high and the great amount of released heat causes an accelerated degradation of the catalyst and a loss of its activity.
It has been proposed to subject the oxygenated gaseous effluents containing nitrogen oxides to a selective reaction with ammonia, the presence of oxygen not interfering with this type of reduction. According to French patent 1,205,311, this reaction is catalyzed by metals of the platinum group or metals of the iron group according to French patent 1,233,712. However, these catalysts as described, after a certain period of use, undergo, in the presence of ammonia and nitrogen dioxide NO.sub.2, a change involving a reduction of their catalytic efficiency.
Further, it is known to submit the oxygenated gaseous effluents containing nitrogen oxides to a selective reduction in the presence of vanadium pentoxide deposited on alumina, especially from French patent 1,412,713. However, catalysts comprising a transition metal oxide as an active component do not promote the reduction of nitrous oxide (N.sub.2 O).
In the technical field of purification of gaseous effluents polluted by nitrogen oxides, by selective reduction of said oxides with ammonia leading to the production of nonpolluting nitrogen and water products, French patent 2,413,128 describes catalytic compositions containing as active elements, iron and chromium in the form of oxides associated with alumina and at least one promoter selected from rare earth oxides alone or in combination and rare metals accompanying platinum ores. These catalysts are applicable with satisfactory results in a purification process at a temperaure between 120.degree. and 350.degree. C.
It was desirable to extend the zone of using the process of reduction of nitrogen oxides to the higher temperatures. Actually, in many cases the effluents to be treated had a temperature greater than 300.degree. C.; and it was advisable to avoid resorting to heat exchangers to cool then heat the gases to achieve a gain in investment and a saving in operation.
Further, the fact of performing the selective reduction by ammonia at higher temperature makes possible the purification of effluents containing, besides nitrogen oxides, sulfur oxides; these latter do not combine, at this temperature level, with ammonia to form ammonium sulfate which causes a deactivation of the catalyst.
French application 2,568,789 proposed a reduction process, denitrification of ammonia of residual gases in the presence of a zeolite of the type of substitution with hydrogen and/or iron, at a surface velocity of 5 m/h or less with a ratio of the ammonia NH.sub.3 concentration to the nitrogen dioxide NO.sub.2 concentration of 1.3 or more, in particular 1.6 or more.
The catalyst, seeming to have been tested only on a laboratory scale, was prepared by immersion of a natural or synthetic zeolite in hydrochloric acid, an aqueous solution of ferric chloride or an aqueous solution of ferric nitrate Fe(NO.sub.3).sub.3, then calcination at about 500.degree. C. The tests were conducted in the presence of mordenite with substitution with H, Fe obtained by replacing 42% of equivalent of Na, K and Ca contained in the mordenite with hydrogen and 35% of equivalent for iron.
A resistant catalyst was sought making it possible to obtain equivalent purification performances in extensive fields of application and at all nitrogen oxides contents, by using a lower NH.sub.3 /NO.sub.x ratio, while performing the reaction of reduction of nitrogen oxides at high temperature and at volumetric speeds of circulation of gases which can be high.